The answer to this question will depend on the particular girl and even where she is in her monthly cycle. On my wife there are times when I need to spread her lips some in order to get myself in her. Other times, depending on how much and what type of foreplay I will slip right in. Usually if I have performed oral sex, I have already spread them to get to the sensitive clit. Take your cue from how easy your able to find her vagina with your penis. If you run into obstruction gently use a hand to spread the lips a little.

Nah....When two people are ready for sex and everything is lubed properly the lips will separate without a problem. The vaginas on some girls when excited actually swell and the lips spread without touching them. That's a very good indication that she is ready for sex. It's a beautiful sight!